Inspired by the effect of power supply voltage sag in tube rectified class AB amplifiers, Whalestone developed a distortion circuit featuring signal amplitude dependent dynamic headroom.
The design goal was not to emulate a certain amplifier, but to achieve tones incorporating traits associated with pre- and power amplifier distortion in some tube amplifiers exhibiting power supply sag.

Class AB amplifier sag

Simple class AB amplifier block diagram

fig.1 Simple class AB amplifier

A simple class AB amplifier consists of a preamplifier, a power amplifier featuring a phase inverter and push/pull output stage, and a power supply featuring a rectifier and filters (fig.1).
The internal resistance of a tube rectifier yields, along with filtering, a current dependent and in class AB hence signal amplitude dependent power supply voltage drop known as sag. This results in dynamically reduced headroom of both the positive and negative output swing of the power amplifier.
Sag also influences asymmetrical tube preamplifier distortion, with a slower response due to additional preamplifier power supply filtering.

Whalestone Jade sag

Whalestone Jade sag function block diagram

fig.2 Whalestone Jade sag function

The Whalestone Jade sag function employs two gain stages controlled by sag control circuitry (fig.2).
The sag control circuitry processes the output of the second gain stage to generate a set of sag control signals which direct gain stage headroom manipulation.
The first gain stage produces asymmetrical distortion controlled by a slowed down sag control signal, echoing sag condition characteristics of some preamplifiers. In the second stage both the positive and negative clipping thresholds are controlled by two respective sag control signals, emulating some class AB power amplifier sag condition characteristics while avoiding crossover distortion.